[X86] Deleting README-MMX.txt now that all tasks have been completed.

MMX buildvectors were improved at rL327247 - new MMX bugs should be raised on bugzilla

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327300 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed